This invention relates to methods for fabrication of perforated composite panels, and particularly such panels which are conformed to curved surfaces.
Perforated panels are used in various applications for sound suppression or laminar flow control. Acoustic noise control or sound suppression may be accomplished by acoustic labyrinths, in which the energy of an unwanted sound wave is dissipated. One easily fabricated form of such labyrinths is a plurality of closed-end apertures in a surface facing the acoustic source. Laminar flow control may be accomplished by porous or perforated surfaces through which turbulent air adjacent the surface is vacuumed, to leave laminar flow at the surface. When such perforated panels are used in vehicles, they must conform to the shape of the vehicle, which, in most cases, includes surfaces which are curved in two or in three dimensions. Vehicular use has the further requirement of low weight, so perforated panels are ordinarily made from fiber reinforced laminated polymer composites.
Perforated panels of fiber reinforced laminated polymer with apertures can be fabricated with the aid of a studded mold or pin mandrel. Resin impregnated fabric is pressed over the studded tool and cured to create the perforations. The curing often requires high temperatures, so the tool must have a high temperature capability. A mold release agent must be applied over the mandrel to aid in release of the mold from the cured composite without damaging the composite. Cleaning of the mold release agent from the perforated panel is labor intensive and costly. When the cured panel is contoured or curved, it may be necessary to use a mandrel with rubber or elastomeric studs or pins, to allow them to be removed from the cured panel.
Improved methods are desired for fabrication of perforated fiber reinforced laminated polymer panels or sheets.
A sheet of uncured polymer, which is preferably reinforced with multiple layers of fiber, is staged or partially cured to a state in which the sheet is rigid at room temperature. Multiple apertures are drilled into, and preferably through, the sheet. The apertures are for acoustic or laminar flow control, and include holes having diameters in the range of 0.025 to 0.120 inch, although diameters outside these ranges are possible. In the case in which the fabricated panels must conform to a particular shape, the perforated, partially cured panels are heated to a temperature at which the panel softens, and in the softened state are formed in order to conform to the desired surface. The partially cured, perforated sheets are then cured. In order to prevent hole closure (or other major distortion of the drilled holes) during the cure, at least one surface of the sheet is held against an elastomeric tool surface, where the elastomer expands with temperature, and a mold release layer in the form of a fibrous material lying between the tool surface and the sheet allows the cured sheet to be released from the tool surface. In a preferred embodiment of the invention, the perforated, partially cured sheet is sandwiched between layers of glass fabric during cure on the elastomeric tool surface.
In a preferred embodiment of the method, the polymer is of epoxy or bismaleimide material, and the reinforcement is graphite fiber. The elastomeric tool surface on which the panel is cured is of a fluoroelastomer rubber material laminated with fiber reinforcement for dimensional stability, and with a release coating of tetrafluoroethylene surface film on one side to aid part/liner separation. Part/liner separation is further aided by a glass fabric placed between the sheet being cured and the elastomeric tool surface.